High voltage integrated circuits (HVICs) translate low-voltage control signals to levels that are suitable for driving power switches in high voltage applications. HVICs can also translate signals from higher voltage levels to lower voltage levels in a single chip with high voltage and low voltage circuits, sharing the same I/O pad. As such, voltage isolation must be carefully managed to prevent the low voltage circuit from permanent damages caused by high stress. One of the HVIC designs is to integrate a high voltage resistor in the semiconductor structure for the sake of reducing the level of high input voltage before it enters into the low voltage circuit. Poly-silicon is frequently used in the existing manufacturing process, and the appropriate resistance for a specific application can be tuned by the doping concentration and the total length and pattern of the poly-silicon resistor.
In the case of when a high voltage surge occurs to the I/O pad of the HVIC, the poly-silicon resistor itself can be damaged by the high stress, and the low voltage circuit would be unavoidably impacted due to the lack of stress reduction. The conventional HVIC structure utilizing a poly-silicon resistor may also include an inherent capacitor using the poly-silicon resistor as one electrode and the substrate connecting to ground as another. The built-in capacitor is designed to shunt the high to stress when the high voltage surge exceeds the breakdown voltage of said capacitor. In this case, the poly-silicon resistor can be protected from high voltage burn out.
To permit a higher input voltage applied to the HVIC, a circuit that is allowed to hold a greater breakdown voltage is desired in order to facilitate the function of voltage isolation. The circuit shall be suitable for a specific high voltage application with a structure that properly shunts the high voltage surge and allows the structure to withstand a higher breakdown voltage.